The invention relates to a cathode having a matrix body impregnated with an alkaline earth compound, whose surface is provided with a top coat comprising a high melting point metal, such as particularly tungsten, and scandium.
A cathode of this type is known from U.S. Pat. No. 4,855,637. This patent proposes a top coat provided by sputtering, which consists of single layers of different density comprising tungsten and scandium.
Heated cathodes with top coats comprising scandium have a high emission at relatively low temperatures, for example 100 A/cm.sup.2 at 950.degree. C. When used in vacuum tubes having a high electron emission current load of the cathode, particularly for projection television, HDTV and high-resolution monitors, these cathodes are very suitable due to their high emission. In this respect it is important that the cathodes have a satisfactory resistance or regeneration capability after an ion bombardment which occurs when forming the tube or under poor vacuum conditions. The ion bombardment causes the evaporation of the barium oxygen surface complex on tungsten which is responsible for the high emission of such an alkaline earth dispenser cathode. A satisfactory recuperation after ion bombardment requires a rapid dispensation of the components involved, particularly also of scandium.